1. Field of the Invention
This invention relates generally to adhesive compositions and more particularly to such a composition having special utility in the fabrication of electronic equipment.
2. Description of the Prior Art
Electronic equipment has become more and more economical in weight and smaller in dimension. This in turn calls for printed circuitry of increased compactness and multiplied construction. Glass epoxy prepregs find wide application in assembling or otherwise laminating a dielectric substrate with a copper pattern to thereby form a printed wiring board. Such type of prepreg, derived by impregnating an epoxy resin into fibrous glass, has been found unsatisfactory for reasons given here and below.
To remove air entrained in the prepreg, the foregoing mode of assembly is necessarily effected with a two-stage rise of temperatures and at a pressure as high as 40 kg/cm.sup.2. These conditions will in most cases produce a printed wiring board having residual strain which results from the heat and pressure during lamination and will eventually lead to pattern failure. Alternatively, it has been proposed that a composition made up of an epoxy resin and an acrylonitrile-butadiene rubber (NBR) be used in place of a glass epoxy prepreg. This composition, though able to be laminate at low pressure, is susceptible to insufficient electrical characteristics particularly with time under high temperature and humidity conditions. In such instance the decline in electrical quality is due primarily to the presence of the NBR. That problem is thought to be alleviated by the replacement of NBR with a carboxyl group-containing NBR rubber. NBR rubbers of this type are highly compatible and crosslink with the epoxy resin and hence are capable of improving electrical properties. The last type of composition, however, undergoes gradual reaction even at room temperature and hence has poor stability during storage.